Ventilation for High Occupancy Areas

[supaman79]

Guys,

When ventilating a high occupancy area such as a cafeteria or lunch room...how much can I reduce to per Mass. Energy Codes. The energy code specifies that I have to reduce to the spaces minimum per the Mechanical Code Appendix A. But within Appendix A I can't find the minimum requirement. Also from experience I remember that since the occupancy in such a space is large, but for a short period of time I'm only required to bring in 50% of the required O.A. to the space. Any input guys?

 

[imok2]

In cafeterias and luncheonettes, the air distribution system must keep food odors at the serving counters away from areas where patrons are eating. This usually means heavy exhaust air requirements at the serving counters, with air supplied into, and induced from, eating areas. Exhaust air must also remove the heat from hot trays, coffee urns, and ovens to minimize patron and employee discomfort and to reduce air-conditioning loads. These factors often create greater air-conditioning loads for cafeterias and luncheonettes than for restaurants. Ashrae std.62-1989 recommends 20 CFM/person ventilation.
You want to keep the max CO2 at 500-600 PPM

 

[quark]

500 to 600 ppm is not possible with 20cfm/person in my view. Typical concentration of CO₂ in outside air is 0.03%. CO₂ production by a sedentary occupant who is eating a normal diet is 0.0112 cfm.

So outdoor air required to keep the limits under 500 ppm is 0.0112x100(0.05-0.03) = 56 cfm. (500ppm is 0.05%)

1000ppm and corresponding 16 cfm seems to be ok and I want to listen from Imok2 about this.

Regards,

 

[imok2]

Quark, Outdoor background air CO2 = 350ppm average
ashrae std recommends 1000ppm CO2 max
Human discomfort begins around 800-1000ppm
OSHA and Air Force propose 650ppm max, but when I said 20CFM/person I wasn't saying that you could get at 500/600 with 20Cfm . I was telling the young man he should try to keep his CO2 below an uncomfortable level, sorry if I confused u and thanks for pointing that out.

 

[imok2]

Supaman,
Maybe the best solution for you is to incorporate rotary energy recovery ventilators (ERVs) into your existing system. This equipment preconditions the outdoor air by recovering energy from air being exhausted from the building. The preconditioned outdoor air is then injected into the return duct and supplied to the room spaces using the existing air distribution system. Designing a system with an ERV is similar to a conventional design with two minor differences.
First, the ERV replaces the exhaust fan. Second, outdoor air enters the ERV and is preconditioned by the wheel, rather than entering the air conditioning unit directly. In most cases, the exhaust and supply ductwork do not change. A short run is typically required to duct outdoor air to the return duct and ERVs also work nicely in systems with economizer operation. So energy savings are another benefit that ERVs provide because of lower cooling and heating requirements